In general, the present invention relates to video driver circuits and methods for use with a video display. More particularly, the invention relates to such circuits and methods having associated brightness and contrast controls of particular utility in the color video displays of television and data display units.
In color video display systems such as television receivers and data display units it is necessary to have several signal paths with associated gain controls which must track together. With poor gain tracking between signal paths, an undesirable color error will occur. For example, the gain controls must track such that red, green and blue signals of equal amplitudes give white for all contrast control settings. Moreover, the contrast control of the color difference signals (R-Y and B-Y) must also track accurately.
Previously, in bipolar integrated circuits, gain controls were usually independent current steering devices for each video signal path whose only advantage are their simplicity and small size. However, these individual current steering gain controls have major drawbacks in that V.sub.eb matching errors of only five millivolts can give an error of twenty percent in the collector current ratio. Moreover, the gain control law has a limited linear range.
In the past, it has been generally necessary to provide separate drive controls for each of the three signal paths in a color video display. Further, inasmuch as the respective guns of a CRT picture tube age differently, it has been necessary to adjust the drive controls of the CRT throughout its life, particularly during the first twenty-four hours. In this respect, it has been necessary to "burn-in" the set to stabilize operation of the guns such that a final manual adjustment could be made of the respective gain controls.
In this regard, several integrated circuits for television receivers have introduced automatic setting of the CRT operating point, or its dc (black) reference level. However, there has heretofore been no provision for automatic setting of the video gain or contrast (gray) operating point.
Further, the brightness control of a television or video display receiver is usually accomplished by shifting the bias or dc reference level of the video display. This operation has the effect of changing both the average luminance of the display as well as the contrast ratio of the picture. In like manner, the contrast control is normally an amplitude control, the operation of which will also affect average luminance as well as the contrast ratio. In a color television or data display system, saturation and hue may also be affected. Previously, there has been no provision for compensating the brightness control signal such that the picture black level remains unaffected by the contrast control signal.
It is therefore an object of the present invention to provide an improved video driver circuit and method for automatic gray scale adjustment and elimination of contrast tracking errors.
It is further an object of the invention to provide an improved video driver circuit and method for automatic gray scale adjustment and elimination of contrast tracking errors which provides for the accurate tracking of gain between red, green and blue video signal paths in a video display.
It is further an object of the invention to provide an improved video driver circuit and method for automatic gray scale adjustment and elimination of contrast tracking errors which provides the accurate tracking of color difference signals (R-Y and B-Y) in video display.
It is further an object of the invention to provide an improved video driver circuit and method for automatic gray scale adjustment and elimination of contrast tracking errors which obviates the burn-in requirement of a CRT saving setup time and power.
It is further an object of the invention to provide an improved video driver circuit and method for automatic gray scale adjustment and elimination of contrast tracking errors which allows for the elimination of separate drive controls for the separate video signal paths in a color video display.
It is further an object of the invention to provide an improved video driver circuit and method for automatic gray scale adjustment and elimination of contrast tracking errors which provides for the modulation of the brightness control signal by the contrast control signal such that picture black level does not change with contrast.